Optical fiber amplifiers are widely used for signal amplification in optical data transmitting networks based on wave-length-division multiplexing (WDM). Changes in network configuration, component failures, fiber breaks or protection switching can cause abrupt changes of optical input power. These changes cause fast changes of the average power levels of the surviving channels at the output of the amplifiers. Furthermore, such changes can be transferred to other wavelengths due to nonlinear fiber effects and the non-ideal dynamic properties of erbium-doped fiber amplifiers (EDFAs). These changes can propagate to other sites leading to optical power fluctuations across the whole network and possibly to oscillations. Thus, even channels that are not directly affected by the switching operations or failures can suffer from some performance degradation at the receivers.
Furthermore, gain variations can also accumulate in a cascade of amplifiers. Thus, even small gain variations can result in significant power changes at the receivers. Therefore, efficient amplifier control techniques are required that allow to keep the inversion and as a consequence the gain profile of an amplifier or an amplifier stage relatively constant even if the input power changes.
Fast electronic control architectures are currently the most economical solution to stabilize the gain of EDFAs. Commonly, feedback architectures are used since they allow to adjust the gain or output power to given target values and to compensate for control errors. However, purely feedback based controllers cannot meet the transient performance requirements for dynamically reconfigured networks. Fortunately, feedback controllers can be complemented by a feedforward controller. The combination of the two types of controllers provides fast response to any changes with the feedback system cleaning up for any error in the predetermined adjustment made by the feedforward control.
In order to keep the gain variations as small as possible, the feedforward control has to predict as accurately as possible the pump power required to keep the inversion constant for the changed input power conditions. Typically, the required pump power is estimated to be a linear function of the input power plus some offset. However, this approach neglects the influence of wavelength, which can lead to significant deviations between calculated pump power and actually required pump power.
An approach to alleviate this problem is described in the patent U.S. Pat. No. 6,341,034 B1. The described solution consists of an additional monitoring path at the amplifier input comprising an additional optical filter and succeeding means to measure the optical power after the filter. This adds some cost to the amplifier. However, the main drawback of the solution is the increased loss in the signal path at the input of the amplifier due to the increased power that has to be coupled out.
The patent EP 1 695 467 B1 discloses a method for improving the transient performance by a feedforward control taking into account that the required pump power depends on the wavelength of the surviving channels in a drop process. The main advantage of this solution is that this is achieved without adding some additional components to a standard amplifier design. However, the solution adds some complexity to the control software and increases the efforts required for amplifier calibration.
Amplifiers used for commercial applications typically consist of several stages, even if there is no access port between them. In many cases, the stages are separated by an isolator that reduces backward travelling amplifier spontaneous emission and thus contributes to improve noise figure.
In principle, the power required for signal amplification in the different stages could be provided by pumps dedicated to one stage each. However, cost reduction has become a continuing task. Therefore, pump power bypassing or pump power splitting has become a widely used technique to reduce amplifier cost. If pump power splitting is applied to amplifier stages that are separated by a component afflicted with delay such as a dispersion compensating fiber (DCF) unacceptable poor transient performance is gained. Therefore, pump splitting is typically applied only to stages that are all before the DCF or after the DCF.